Existing contact masks used for the selective deposition of metal pads in back-end-of-line (BEOL) integrated circuit manufacturing are approaching the limit of their ability to define smaller features to increase the density of integrated circuit (IC) chip arrays such as memory or logic arrays. In moving to increased integration density, solder deposition and chip attach processes have had to be conducted at lower temperatures than before, in order to reduce the effects of differential thermal expansion between the chip substrate and the mask or chip carrier during processing. In order to permit chip attach processes at lower temperatures, two step deposition processes have been disclosed in the above-referenced U.S. patent application Ser. Nos. 08/476,474 and 08/740,571 (Attorney Docket Nos. FI9-95-068 and FI9-95-164, respectively), in which a low melting point (LMP) capping composition (e.g. tin or a eutectic composition of solder metals: tin and lead) is deposited to cover a higher melting point solder feature. However, in practice, the capping step must be tightly controlled to form caps of sufficient volume over the solder features while avoiding the deposition of the capping material at undesired locations on the substrate. As will be understood, the effects of misalignment due to differential thermal expansion between the substrate and the mask or between the first deposited features and the mask must be better controlled.
"Knife edge" sidewalls, i.e. sidewalls having an edge which projects into the aperture opening, are formed in apertures of contact masks used for material deposition. The knife edge is a result of chemical etching from both sides of a contact mask to form apertures therein. Conventionally, a knife edge comes to a sharp edge about halfway up the height of the aperture sidewall. Such knife edge apertures are understood to have beneficial effects in maintaining a separation between the mask and deposited material, up to a height of deposited material of 125% of the mask thickness above the substrate. However, when the height of the deposited material is greater than 125% of the mask thickness, the deposited material can undesirably become detached from the substrate upon removal of the mask. This problem is further exacerbated when the mask is used to define second features over first raised features already deposited through the mask in a previous deposition step, as described in Applicants' co-pending U.S. patent application (Ser. No. 08/476,474). In such case, even slight misalignment of such conventional mask apertures to previously deposited features, whether thermal or mechanical in origin, can lead to the deposited features being gouged or detached from the substrate when the mask is removed, regardless of the height of the features relative to the mask thickness.
U.S. Pat. No. 5,359,928 to Blessington, deceased et al. ("the '928 Patent") describes a paste screen printing stencil having apertures which are larger on a bottom substrate facing surface than on a top paste receiving surface (FIG. 3, col. 2, lines 65-68). The '928 Patent notes that a knife edge in the sidewall of apertures can lead to deposited solder paste being pulled away or smeared upon removal of the stencil (col. 6, lines 31-55). The '928 Patent proposes a solution to such paste screening problem which is very different from Applicants' invention, as will be described in the following. The '928 Patent describes using electroforming instead of etching to form screen apertures to eliminate the knife edge in the sidewall and to provide aperture sidewalls that are tapered and smooth. The '928 Patent further describes forming raised edges 13 around the bottom aperture openings for use in sealing the screen stencil against leakage of the liquid paste around deposited features on the substrate.
As will be understood from the description of the invention to follow, the screen printing stencils described in the '928 Patent could not be used in the manner described herein for the invention in forming solder features by vapor deposition onto semiconductor wafers. Electroformed panels are known to have much less shear strength than etched panels of rolled or forged metal. For this reason, electroformed panels deflect and can break too easily for use in a vapor deposition process as described in the following. Secondly, the raised edges surrounding the mask apertures described in the '928 Patent can place stress in semiconductor wafer substrates, causing indentation of polymer films and potential cracking and breakage of inorganic dielectric films on such substrates. Thus, it will be understood that the teachings of the '928 Patent cannot be meaningfully applied to the problem solved by the invention in depositing materials onto a wafer substrate.